Various types of warning devices have been proposed to indicate whether bearings in dynamo electric machines are worn. The accuracy of operation of bearings in automotive alternators is particularly important due to the relatively small air gap with which these machines operate. In previously proposed warning devices the wobble effect was utilized that is, bearings have increased play therein shortly before failure. Warning devices therefore have been utilized in which the bearing play can be sensed and, if the bearing play increases above a predetermined value, an indication of incipient malfunction is provided. Such warning devices are difficult to install, particularly in the compact structures of automotive alternators in which bearing play which leads to damage is very small, so that recognition of damage likewise requires sensing extremely small deviations from a predetermined tolerance level.